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How Large Should My Furnace Be?

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How Large Should My Furnace Be?

Posted On: November 13, 2024

How Large Should My Furnace Be?

The size of a furnace is typically measured in BTUs (British Thermal Units), and the right size depends on various factors such as the size of your home, your climate zone, insulation quality, and the number of windows and doors. Here's a breakdown of what to consider when determining the appropriate furnace size:


1. Square Footage of Your Home

  • Generally, you can calculate the BTUs needed based on your home’s square footage. As a rule of thumb, most homes need between 30 to 60 BTUs per square foot.
  • For example, if your home is 2,000 square feet, a rough estimate for furnace size would be
  • 2,000 sq ft×40 BTUs per sq ft=80,000 BTUs2,000sq ft×40BTUs per sq ft=80,000BTUs

2. Climate Zone

  • Warmer climates require fewer BTUs per square foot, while colder climates need more.
  • Here’s a rough guide by climate:
    • Warm climates (like Florida): 30–35 BTUs per square foot.
    • Moderate climates (like the Midwest): 40–45 BTUs per square foot.
    • Cold climates (like New England): 50–60 BTUs per square foot.

3. Insulation and Windows

  • If your home has good insulation, you might need fewer BTUs. Poor insulation or old, drafty windows can increase the BTU requirements.
  • High-quality windows, doors, and insulation can reduce the furnace size by 10–20%.

4. Ceiling Height

  • Most BTU estimates assume an 8-foot ceiling. If your ceilings are higher, you may need a larger furnace to heat the increased volume of air.

5. Furnace Efficiency

  • High-efficiency furnaces produce more heat per BTU, so you can often go with a smaller size if you’re choosing a furnace with an efficiency rating of 90% or higher.

Example Calculation:

Let’s say you have a 2,000-square-foot home in a moderate climate with good insulation and standard 8-foot ceilings. You might aim for around:


2,000 sq ft×40 BTUs per sq ft=80,000 BTUs2,000sq ft×40BTUs per sq ft=80,000BTUs


If you choose a high-efficiency furnace (like 90% AFUE), you could reduce the size to around 72,000 BTUs.


When in Doubt, Consult a Professional

A qualified HVAC contractor can do a more precise calculation called a Manual J load calculation, taking into account all relevant factors, and recommend the right furnace size. This prevents you from oversizing or undersizing, which can lead to inefficiencies and comfort issues.



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See What Our Customers Are Saying:

by Trevor Owens on Twintech Heating and Cooling

I returned from vacation to find my AC unit on the Fritz. After calling way too many places I connected with TwinTech. I’ve used the company in the past for my furnace so I was happy when I recognized Greg at my front door. He’s great and the service is always top notch. I’d highly recommend using twintech for honest, superior service in the Durham region. Thanks!

by Nicole Alain on Twintech Heating and Cooling

I called Twin Tech when my air conditioning stopped working. Greg came over to diagnose the issue - the same day. He was able to diagnose the issue and resolve it quite quickly. Although it was a quick fix, Greg was very thorough in explaining the issue and provided great advice. Not to mention, I felt very comfortable having Greg in my home - which I consider very important when hiring a service provider. Twin Tech will be my go-to from now on. Thank you!!

by Courtney Parkes on Twintech Heating and Cooling

Just hired TwinTech Heating & Cooling to do an A/C Unit removal & reinstall to allow for landscaping access on the side of my house, and they were wonderful!
Bob, Greg & Mitch make a great team! Quick appointment times, super knowledgeable, expert troubleshooting capabilities & very customer service oriented.
I would highly recommend them for all of your HVAC needs! Great job guys!!

by Jacqueline Graham on Twintech Heating and Cooling

Twintech Heating & Cooling installed my furnace in 2022 and in fall of 2023 installed my Panasonic Heat Pump. Greg and Mitch did a great job. Helped me through the rebate process. Super happy with the Heat Pump. Only used my furnace 5 times this winter. Energy bills lower, carbon emissions lower.Thank you Twintech. I recommend highly!

by Bill Grainger on Twintech Heating and Cooling

Just had a new furnace installed by Twintech. I can’t express how great these guys are at what they do. From the sales to installation they were professional all the way. Very reasonable pricing and courteous during the installation and willing to answer any questions I had. Highly recommended !!!

by Colin Vernon on Twintech Heating and Cooling

They've cleaned my furnace a couple of times and mentioned that the hot water tank standards were going to change soon so I decided to change mine out.

There were a few challenges during the process but the installers handled the problems without missing a stride! I now have really hot water again!

by Heidi B on Twintech Heating and Cooling

Highly recommend Twintech Heating company, we got honest and detailed information on what was needed to replace and install a new gas furnace. Bob and Greg made us feel comfortable withe process through their thorough explanation and knowledge of the right furnace for our needs. The professionalism and courteous friendly attitude towards the customer is what we liked the most as they treated us like family. So happy with our decision and new furnace we will be looking into get our rental water heater replaced and installed by Twintech team. We feel better having their knowledge on our side and will only trust them with any further maintenance of furnace.

by Kirushanth Rasiah on Twintech Heating and Cooling

Customer service and knowledge is priority in this industry, and Twintech has it. I've dealt with their team for maintenance and a furnace replacement. They're accommodating and make great recommendations, and came through at a great price over the big guys in the industry. They'll be my next call when I need service or maintenance.

by Leonard Glass on Twintech Heating and Cooling

I decided that the noises and age of the existing furnace which was old and not very efficient to replace it. Gave Greg a call and Grant and Greg got to work on it. Because of supply issues with the current heating type that would be an easier install Greg convinced me to change the location of the system and it would provide me better service for heat and efficiency. I had skeptical but went ahead anyway putting trust in Greg And Grant. Greg, Grant and Mitch came through and in 2 days had completely relocated my heating and A/C system. I can honestly say Greg was right my old house has been warmer and more evenly heated as it ever was. For anyone reading this rather long winded review can take my word for it, Twin Tech can take care of your needs.

by Nicole Alain on Twintech Heating and Cooling

I called Twin Tech when my air conditioning stopped working. Greg came over to diagnose the issue - the same day. He was able to diagnose the issue and resolve it quite quickly. Although it was a quick fix, Greg was very thorough in explaining the issue and provided great advice. Not to mention, I felt very comfortable having Greg in my home - which I consider very important when hiring a service provider. Twin Tech will be my go-to from now on. Thank you!!

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See What Our Customers Are Saying:

by Trevor Owens on Twintech Heating and Cooling

I returned from vacation to find my AC unit on the Fritz. After calling way too many places I connected with TwinTech. I’ve used the company in the past for my furnace so I was happy when I recognized Greg at my front door. He’s great and the service is always top notch. I’d highly recommend using twintech for honest, superior service in the Durham region. Thanks!

by Nicole Alain on Twintech Heating and Cooling

I called Twin Tech when my air conditioning stopped working. Greg came over to diagnose the issue - the same day. He was able to diagnose the issue and resolve it quite quickly. Although it was a quick fix, Greg was very thorough in explaining the issue and provided great advice. Not to mention, I felt very comfortable having Greg in my home - which I consider very important when hiring a service provider. Twin Tech will be my go-to from now on. Thank you!!

by Courtney Parkes on Twintech Heating and Cooling

Just hired TwinTech Heating & Cooling to do an A/C Unit removal & reinstall to allow for landscaping access on the side of my house, and they were wonderful!
Bob, Greg & Mitch make a great team! Quick appointment times, super knowledgeable, expert troubleshooting capabilities & very customer service oriented.
I would highly recommend them for all of your HVAC needs! Great job guys!!

by Jacqueline Graham on Twintech Heating and Cooling

Twintech Heating & Cooling installed my furnace in 2022 and in fall of 2023 installed my Panasonic Heat Pump. Greg and Mitch did a great job. Helped me through the rebate process. Super happy with the Heat Pump. Only used my furnace 5 times this winter. Energy bills lower, carbon emissions lower.Thank you Twintech. I recommend highly!

by Bill Grainger on Twintech Heating and Cooling

Just had a new furnace installed by Twintech. I can’t express how great these guys are at what they do. From the sales to installation they were professional all the way. Very reasonable pricing and courteous during the installation and willing to answer any questions I had. Highly recommended !!!

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